Phased-array and similar antennas generate antenna beams determined by the relative phase and amplitude relationships of signals that excite numerous antenna elements arranged in an array. Adjustable phase shifters and amplitude adjusters are often used in combination with a beamforming network, such as a Butler matrix or the like, and a corporate feed network to control the collective antenna beam formed from numerous beamlets associated with the numerous antenna elements. The adjustable phase shifters permit electronic steering of the antenna beam.
In a typical arrangement used in the communication of digital signals, a digital input signal is converted into an analog baseband signal that is mixed with an intermediate frequency (IF) signal, amplified, mixed with a radio frequency (RF) signal, and then amplified again to produce a communication signal. The communication signal excites one or more antenna elements after being routed through a corporate feed network that may include one or more beamforming networks. However, this arrangement leads to undesirable effects which are related to the use of an analog baseband signal.
The analog baseband signal must be processed through numerous devices before the resulting communication signal is formed. The use of a greater number of devices to perform a function which may otherwise be accomplished with fewer devices is undesirable because it usually imposes power consumption, reliability, weight, and system complexity penalties. The use of linear analog amplifiers necessitated by the use of an analog baseband signal causes conventional techniques to exhibit a particularly poor power efficiency parameter.
In addition, the use of analog baseband signals excessively complicates the layouts of feed networks that drive array antennas. In a conventional arrangement, a large number of communication signals are generated and propagated through the feed network to a large number of antenna elements. The parallel propagation of numerous communication signals takes place while preserving relative phase relationships between the numerous communication signals. The constraint of routing numerous transmission lines while maintaining relative phase relationships imposes a severe burden upon the geometry and layout of an array antenna.
The layout burden is exceptionally severe when imposed upon beamforming networks. As the frequencies at which RF communications take place increase, the array antenna geometry shrinks. This forces the layout constraints to be observed in smaller and smaller areas. Due to layout constraints, array antennas and associated beamforming networks for large numbers of elements become impractical when designed using conventional techniques for operation at higher frequencies.
Accordingly, a need exists for an array antenna transmitting apparatus that is not driven by a conventional analog baseband input signal.